As the structure sizes of modem semiconductor components continue to decrease, greater care must be taken to ensure proper environmental conditions during manufacturing processes. In particular, extremely stringent cleanliness requirements must be ensured. This of particular importance in processing chambers, in which plasma-aided coating or etching processes are performed under vacuum conditions. The presence of even minor impurities can produce significant defects in the layers on a substrate which are produced by plasma chemical vapor deposition (“CVD”), or else may result in faults caused during plasma etching processes. The presence of impurities in a processing chamber may also have additional negative effects on the quality of plasma CVD processes.
Impurities may be residues from previous processing steps in the processing chamber, such as deposits on the inner walls of the processing chamber resulting from coating processes such as plasma CVD processes. These deposits may result in contaminant particles in the processing chamber which may interfere with subsequent process steps. One such processing chamber for carrying out plasma etching processes on individual substrates, i.e., wafers, is disclosed in U.S. Pat. No. 4,602,981 A with relatively large processing chambers being used in order to increase throughput by processing a greater number of wafers at the same time. However, the problem described above can apply to all types of plasma apparatus and CVD apparatus.
DE 196 05 136 A1 describes a method and apparatus for controlling the coating of surfaces in the vacuum chamber. This is done by temporarily separating a process area within the processing chamber and performing a plasma process in that area. This limits the influence of the plasma to the temporarily separated process area so that process gases, the reaction products, and the plasma are isolated to act only on small surfaces.
Using methods such as those described above, contaminating layers may be restricted to grow on a small number of surface areas. The corresponding parts may then be cleaned or replaced during the course of the normal maintenance cycles. One disadvantage of this solution is the rather high level of technical complexity and the large number of components which are additionally required in the processing chamber some of which are also moving components. Furthermore, these additional components may themselves be sources of impurities.
For these reasons, the processing chambers used in presently known manufacturing processes must be cleaned or tuned by means of a cleaning process based on plasma cleaning etching after each plasma processing procedure or after a predetermined number of plasma processing procedures. However, care must be taken during this cleaning process to ensure that no overetching occurs. Overetching would result in degradation of the inner metal surfaces of the processing chamber, which is composed of metal (e.g., Al). Such degradation could lead to considerable reduction in the useful life of the processing equipment. Therefore, while adequate cleaning of the processing chamber is important, overetching must be avoided.
U.S. Pat. No. 5,812,403 describes an apparatus for depositing a dielectric film on a substrate by means of a plasma CVD process performed in a processing chamber. This reference also describes a method for periodic cleaning of the processing chamber using a remote plasma and specific cleaning gases. The remote plasma system produces fluorine radicals which are passed to the processing chamber where they may be used to clean the chamber at high temperatures. This cleaning process is based on chemical reactions—physical sputtering effects are negligible since the remote plasma remains outside the processing chamber.
In contrast, an in-situ plasma system can lead to sputtering effects which can damage the aluminum chamber walls.
U.S. Pat. No. 5,812,403 describes a method for determining the end point (clean endpoint) of a cleaning process which is performed within a processing chamber. In this method, the change in the light absorption of the cleaning gas reactants, for example of SiF4, CF4, C2F6, C3F8, SF6 is observed visually. Reactants are supplied such that they pass an IR light source so that the attenuation of the IR radiation through the reactants can be measured via an IR receiver. If a light intensity reference value is exceeded, the gas supply is stopped and the cleaning process is thus terminated.
One disadvantage in this case is that the hardware complexity is still considerable. Furthermore, there may be significant measurement uncertainty since the optical characteristics of the quartz window which is required for observation may also change during a plasma-chemical coating process. Additionally, it is very difficult to determine the end point by optical means as a result of the minor measurement differences involved, and the corresponding difficulty in converting the optical data to data that can be evaluated electrically. As a result of these difficulties, overetching is generally carried out deliberately in order to ensure that the processing chamber has been sufficiently well cleaned.
Finally, the already cited U.S. Pat. No. 4,602,981 describes a method for controlling plasma etching of wafers in a wafer reactor. In this method, the RF voltage which is influenced directly by the plasma potential is monitored on one electrode of the reactor. This assumes an electrically balanced reactor, so that the change in the plasma density resulting from the emission of secondary electrons within the reactor can be monitored by means of the RF voltage. However, this type of monitoring cannot be used for determination of the endpoint for cleaning etching, since the method control is different.
It is an object of the present invention to provide a method for reliable determination of the endpoint during cleaning etching of processing chambers.